This invention relates generally to length- or angle-measuring instruments for determining the relative displacement between two objects, in which a measuring scale that is connected with one object is scanned by a scanning unit connected to the other object. More particularly, this invention relates to such a scanning unit which is pivotably connected to a mounting member by means of a joint having an axis of rotation running perpendicular to the division plane of the measuring scale. Such measuring instruments are widely used for measuring relative movement between movable machine parts, for example, in machine tools or measuring machines.
In West German GM No. 75 06 036 there is described a digital length-measuring instrument in which a scanning unit, including a grid scanning plate, an illuminating system and photo-sensitive elements, is pivotably mounted to rotate about an axis running perpendicular to the division plane of a measuring scale. In this instrument, the pivotable mount includes a roller borne in V-grooves. After the scanning unit has been properly adjusted, it is fastened unshiftably to a mounting member by means of an adhesive. In order to adjust the position of the grid scanning plate with respect to the measuring scale, an adjusting screw is provided on each side of the axis of rotation of the scanning unit. This arrangement has the disadvantage that, during the adjustment of the grid scanning plate with respect to the measuring scale, the distance between the grid scanning plate and the measuring scale can change. This can degrade the performance of the measuring instrument, because diffraction and imaging considerations require that a precise separation be maintained between the grid scanning plate and the measuring scale.